To harvest energy via a thermoelectric device, the thermal energy must be passed through a thermoelectric conversion device. Conventionally, the thermoelectric generator is provided a low thermal resistance so that heat is transferred from a high temperature source to a low temperature heat sink with the most energy per ΔT (temperature difference between the hot and cold sides of the thermoelectric generator) being harvested. It is known that a heat sink such as that provided by the earth (in terrestrial applications) is large with respect to its heat sinking capability, but high efficiency applications require consistent heat sinking capability without regard to depth, variable soil types, and degree of hydration.
Some implementations, particularly for space and extraterrestrial situations, have made use of the fact that once the sun is occluded, the ambient temperature becomes significantly colder than the heat storage element. Electric energy is generated as the thermoelectric generator is, in effect, run in reverse and the heat energy stored in the heat storage element is exhausted to the ambient. An early patent to Gomez, U.S. Pat. No. 4,251,291, discloses a latent heat storage medium that stores heat collected by an absorber plate during the day and releases the stored heat during the night from the same absorber plate. During both parts of the thermal cycle, the heat flow is directed through thermopiles for generation of electricity.
U.S. Pat. No. 6,914,343 to Hiller et al. discloses a thermoelectric generator that employs a phase change mass as a thermal storage element with a thermoelectric module disposed between the thermal storage element and the environment. During the hot part of a thermal cycle, heat is taken from the environment and routed through the thermoelectric element to the thermal storage element, where the phase change mass is partially or completely melted. During the cold portion of the thermal cycle, heat is extracted from the thermal storage element, passed through the thermoelectric module, and dissipated to the environment. The phase change mass is partially or completely frozen at the conclusion of the cold portion. However, the temperature of the phase change mass is expected to remain relatively constant during both parts of the thermal cycle. An electric circuit enables the electric current produced by the thermoelectric generator to flow in a single direction to an electric storage device, regardless of the direction of heat flow through the thermoelectric generator.
U.S. Patent Application Publication No. 2005/0115600 by DeSteese et al. discloses a thermoelectric power source that utilizes two regions exhibiting a temperature difference between the regions. A thermoelectric device exploits the temperature gradient across the device to generate an electric current. DeSteese et al. describe a thermoelectric generator that employs discrete columnar thermocouple elements disposed in a cross-planar configuration, which has a low voltage output characteristic of a low length to cross sectional area ratio (L/A=λ) of the columnar thermocouple elements. Heat flux is introduced perpendicular to the plane of cross section of the columns. A step-up voltage converter is shown to be necessary to satisfactorily employ a cross-planar configuration when the temperature gradient is small. However, such small temperature gradients have not heretofore enabled efficient conversion of heat energy to electrical energy
DeSteese et al. prefer a plethora of thin film thermocouples with a high L/A ratio disposed on a film substrate in an in-plane configuration. This configuration generates a higher voltage with small temperature gradients. A thin film thermoelectric generator is used by DeSteese et al. and includes alternating strips of n-type and p-type semiconductor material connected in electrical series. Heat flux is caused to flow in the direction of the plane of the thin film.
The λ ratio of an in-plane configuration determines both the thermal and the electrical characteristics of the in-plane thermoelectric generator. See McCarty, Marlow Industries, Inc., Energy Harvesting for Wireless Sensors, www.marlow.com/applications/power-generation/energy-harvesting.html (2010). The ratio between the thermal resistance of the thermoelectric generator, RTEG,th, and the remainder of the system thermal resistances, primarily hot side thermal resistance, HSR, and cold side thermal resistance, CSR, is given by:
                    m        =                              R                          TEG              ,              th                                            HSR            +            CSR                                              eq        .                                  ⁢        1            
The electrical resistance of a single in-plane thermocouple element (of the Plethora of in-plane thermocouples disclosed by DeSteese et al.) is given by:RTEG,el=λ(ρp+ρn),  eq. 2
where ρp is the electrical resistance of the p-type semiconductor, and ρn is the electrical resistance of the n-type semiconductor.
Maximum power is realized from an in-plane thermoelectric generator when the resistance of the in-plane thermoelectric generator and the load resistance are matched. That is, for a selected number, N, of in-plane thermocouple elements and a selected load resistance (Rload), the ratio n=1 where:
                    n        =                              R            load                                N            ·                          R                              TEG                ,                el                                                                        eq        .                                  ⁢        3            
and when the thermal resistance, m=1.
With these conditions met, the output voltage is:
                    V        =                              N            ⁢                                                  ⁢                          α              ⁡                              (                                                      T                    source                                    -                                      T                    ambient                                                  )                                              4                                    eq        .                                  ⁢        4            
where α is the Seebeck coefficient.
In-plane and cross-plane thermoelectric generators are further disclosed in U.S. Pat. No. 7,626,114 to Stark. A cross-plane configuration is shown in FIG. 1, and an in-plane configuration is shown in FIG. 2. Stark discloses a cross-plane thermoelectric generator 101 that offers a relatively low electrical resistance due to a conventionally small number of thermocouples 103, which are coupled by metal contacts 104 in an electrically series orientation and a small ratio of length to cross section area of n-type 105 and p-type 107 semiconductor columns that generate electricity from a thermal differential. The low resistance results in a high current output capability with low output voltage. This voltage becomes sufficient to drive a step-up voltage converter when a large heat flux is passed through the plane of the cross section area. Stark also discloses an in-plane thermoelectric generator 201, as shown in FIG. 2, in which a large number of long and thin n-type and p-type thermoelectric legs, 203, 205 respectively, are disposed parallel to each other on a substrate and electrically connected together in series via metalizations 206. The ratio of the length to thickness of the semiconductor legs results in the amount of heat flowing (in the plane of the substrate 207) through the in-plane thermoelectric generator being relatively small. Moreover, the large number of thermocouple elements connected in series and the ratio of length to thickness of the legs results in a high electrical resistance and a low power output. The depiction of the in-plane thermoelectric generator of FIG. 2 illustrates one p-type and one n-type thermocouple. Conventionally, the large number of thermocouples is rolled up into a cylindrical shape for a thermoelectric generator.
Stark discloses that one cross-plane and one in-plane thermoelectric generator may be operated together with a suitable power management apparatus. The voltage converter of Stark is designed to convert the low voltage, high power output from a cross plane thermoelectric generator into a higher voltage using voltage multiplication in the manner of a charge pump. Thus, the higher voltage needs of the other energy management systems, and the voltage converter, itself, are met. Stark's apparatus, using the same heat source and heat sink for both thermoelectric generators, does not provide the highest efficiency in harvesting all of the available heat energy, particularly when a day-night thermal cycle is involved.
When a day-night thermal cycle is considered—where the day part of the cycle provides a relatively large thermal flux and the night part of the cycle provides a relatively small thermal flux in the opposite direction (from heat energy stored in a thermal storage)—a problem of efficiently utilizing both the high level of heat flux and the low level of heat flux persists in the current technologies. It would be desirable that a technique be devised to optimally convert the high level and the low level heat flux into electric power. A thermoelectric generator is needed that can efficiently generate high electric current as the heat energy is stored in a heat energy storage element. A thermoelectric generator is also needed that can discharge heat energy and generate a relatively high voltage with the low heat flux experienced during the discharge of the heat energy storage during the cold part of the thermal cycle.
Electric circuitry has been devised to operate with this bidirectional flow of heat. However, when the heat flux from the storage element to the ambient is low, as would occur in a terrestrial day-night cycle, the efficiency of the electric circuitry has proven to be undesirably low because the magnitude of the voltage supplied is a small value for conventional thermoelectric generators. Accordingly, it would be desirable to generate the highest open circuit voltage possible, particularly during the cold portion of the thermal cycle, thereby improving power conversion efficiency in a thermoelectric generator.